The Applicant's co-owned U.S. Pat. No. 7,739,941, issued Jun. 22, 2010, discloses a hydraulic drive system for a reciprocating piston pump and a method of effectively controlling the reversal of piston movement at the end of each piston stroke, without the use of position sensors, flow rate sensors or special flow-sensing valves. A shuttle valve arranged in the piston of the hydraulic drive section is activated to open as the piston approaches either end of the hydraulic cylinder head. A pressure drop occurs on the high pressure side of the hydraulic circuit due to the opening of the shuttle valve. A pressure sensor is employed to detect this pressure drop such that the flow of hydraulic fluid can be switched to reverse the movement of the hydraulic piston.
In one application the reciprocating piston pump is a cryogenic pump that pumps gaseous fuel to a high pressure for delivery to an internal combustion engine. The gaseous fuel is pumped through a vaporizer where it undergoes a transition from the liquid to either the supercritical or gas states, and is sufficiently pressurized such that it overcomes in-cylinder pressure when it is directly introduced into combustion chambers of the engine. These applications are referred to as high pressure systems.
In other applications, gaseous fuel is introduced to combustion chambers of an engine by injecting it upstream of intake valves that regulate flow of air to the combustion chambers. In comparison to the described high pressure systems, these applications can be referred to as low pressure systems, referring to the pressure at which the gaseous fuel is delivered to the engine. Fuel injectors inject the gaseous fuel into the intake manifold, where the pressure is substantially less than the in-cylinder pressure during direction injection for the high pressure application discussed above.
The method of detecting the end of stroke for the hydraulic piston in the cryogenic pump, by detecting the pressure drop due to the opening of the shuttle valve, is more challenging in a low pressure system compared to a high pressure system. The relative magnitude of pressure fluctuations caused by shuttle valve switching, and the subsequent pressure drop, is not as distinct compared to the magnitude of hydraulic fluid pressure during compression strokes while pumping gaseous fuel.
FIG. 1 illustrates a trace of hydraulic fluid pressure for a compression stroke and a suction stroke of a reciprocating piston cryogenic pump where the hydraulic fluid is employed to drive a hydraulic piston. Cryogenic fluid is drawn into a pumping cylinder during the suction stroke, and compressed during the compression stroke. Rising edge 10 is the hydraulic fluid pressure during the compression stroke. As the gaseous fuel is compressed the hydraulic fluid pressure increases. Falling edge 20 occurs when the shuttle valve opens as the hydraulic piston reaches the end of compression stroke. The subsequent rising edge 30 represents the pressure drop across the shuttle valve. Flat edge 40 is the hydraulic fluid pressure during the suction stroke where the fluid pressure exerted on the hydraulic piston is substantially constant while cryogenic fluid is drawn into the pumping cylinder. Rising edge 50 occurs when the shuttle valve opens as the hydraulic piston reaches the end of suction stroke. Similar to rising edge 30, rising edge 50 represents the pressure drop across the shuttle valve. As can be seen by the relative magnitudes of the rising and falling edges, the magnitude of hydraulic fluid pressure during compression strokes is comparable to the pressure drop across the shuttle valve during switching events, which makes the distinction between these events more challenging.
The state of the art is lacking in techniques for improving the detection of end of piston stroke in a reciprocating piston hydraulic motor. The present method and apparatus provide a technique for improving the detection of end of piston stroke in a reciprocating piston hydraulic motor that can be employed in both low pressure systems and high pressure systems.